In the prior art, the spanner with at least one strain gauge has the strain gauge being installed at a weak portion of the spanner, referring to FIG. 4. Thus, the strain gauge will measure the deformation of the spanner. The deformation will make a bridge circuit unbalance so as to change a resistance of the strain gauge. Thereby the deformation will change into electric signals to be displayed on a display. An alarm unit is installed in the spanner for alerting the users to know whether an overlarge force is applied to the spanner and thus the user can be alerted and thus the user can screw an object to a predetermined position.
However in the prior art only one alarm unit is installed at a wide surface of the handle. The display and the alarm unit are installed in an output device. However the output device is at a position which is generally held by users. Thus the hand of the user will shield the alarm unit, or if the spanner is operated in a space which is not easily viewed by the user, the user cannot view the alarm and thus the alarm lose its effect. Thereby the output device is held by user and thus the buttons on the output device will be operated by mistake. Further the user's hand will feel ache.
Thus above mentioned prior arts have many defects which are necessary to be improved.